Effect on protein digestibility of different processing conditions in the production of fish meal and fish feed

Journal of the Science of Food and Agriculture J Sci Food Agric 83:775–782 (online: 2003)DOI: 10.1002/jsfa.1396

Effect on protein digestibility of differentprocessing conditions in the production of fishmeal and fish feedJohannes Opstvedt,1∗ Einar Nygard,1 Tor A. Samuelsen,1 Giorgio Venturini,2

Umberto Luzzana2 and Harald Mundheim1

1Norwegian Institute of Fisheries and Aquaculture Research, Dept, SSF, Kjerreidviken 16, 5141 Fyllingsdalen, Norway2ASA srl–Agridea, Servizio Nutrizione, Viale del Lavoro 45, San Martino Buon Albergo, 37035 Verona, Italy

Abstract: The effect of processing conditions on protein digestibility and fluorodinitrobenzene (FDNB)-reactive (available) lysine in the production of fish meal and extruded fish feed has been studied underpilot and commercial conditions using mink as model animals. Fish meal produced under pilot-plantconditions at processing temperatures below 70–80 ◦C (FM1) had protein digestibility of 929 (grams ofprotein digested per 1000 g protein consumed) compared with 905 when processed at temperatures above100 ◦C (FM2). A low-temperature-processed commercial fish meal (CFM1) had protein digestibility of940 compared with 888 for a standard commercial fish meal (CFM2). Pilot-produced extruded fish feedhad protein digestibility of 913 when based on FM1 as the main protein source (95% of total protein)compared with 892 when based on FM2. Commercial extruded fish feed had protein digestibility of 912when based on CFM1 compared with 871 when based on CFM2. Varying extrusion conditions at the pilotscale, ie temperatures from 100 to 126 ◦C and moisture contents from 21 to 12%, did not affect proteindigestibility. Similarly, under commercial conditions, variation in temperature from 89 to 110 ◦C andmoisture from 24.5 to 19.5% did not affect FDNB-reactive lysine and protein digestibility. The FDNB-reactive lysine content and protein digestibility of the extruded feed were less than the values calculatedfrom the ingredient mixture before extrusion. Thus, despite different extrusion conditions not givingdifferent FDNB-reactive lysine and protein digestibility, the total process, ie extrusion, drying and oilcoating, caused a reduction. 2003 Society of Chemical Industry

Keywords: processing; extrusion cooking; fish meal; fish feed; protein digestibility

INTRODUCTIONAnimal feeds are subjected to heat treatment in theprocessing of feed ingredients and production ofpelletized complete feeds. Whereas steam pelletingis broadly used to process complete feeds for landanimals and fish reared under extensive conditions,extrusion cooking is the technology commonly usedfor the production of feeds for intensively rearedcarnivorous fish such as Atlantic salmon (Salmo salar),rainbow trout (Oncorhynchus mykiss) and Europeansea bass (Dicentrarchus labrax).

Exposure to denaturation temperatures mayincrease digestibility of native proteins by unfold-ing the polypeptide chain and rendering the proteinmore susceptible to digestive enzymes. On the otherhand, when proteins are exposed to higher tem-peratures, digestibility is reduced1 due to reactionsbetween amino acids and other compounds and intra-molecular reactions between amino acids within the

protein molecule that cannot be split by digestiveenzymes.2

Depending on processing conditions and types ofmaterial, extrusion cooking may reduce or increaseprotein digestibility.3–9 It is difficult, however, todraw conclusions for processing of fish feed frommost previous extrusion studies, since they usedvegetable materials in food systems, whereas feedsfor carnivorous fish are formulated based on fishmeal combined with cereals and vegetable proteins.Moreover, whereas extrusion cooking is usuallythe final processing step in the production offood, the production of fish feeds comprises twoadditional processing steps that also involve the useof heat, ie drying and the addition of oil. There is,therefore, a need to explore the effect on proteindigestibility of modern fish-feed processing technologyinvolving extrusion cooking, fat coating and drying.Furthermore, as low-temperature processing is used

∗ Correspondence to: Johannes Opstvedt, Norwegian Institute of Fisheries and Aquaculture Research, Dept, SSF, Kjerreidviken 16, 5141Fyllingsdalen, NorwayE-mail: [email protected]/grant sponsor: Commission of the European Communities, Agriculture and Fisheries; contract/grant number: CT96-1329(Received 11 July 2002; revised version received 9 December 2002; accepted 14 January 2003)

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to an increasing extent in the production of fish mealfor high-quality fish feed, it is necessary to explorethe combined effect of processing conditions in theproduction of fish meal and fish feed.

The present study was part of an internationalcollaborative project with the aim of studying thecombined effects of processing temperatures in theproduction of fish meal and the processing of fishfeed by extrusion cooking on feed quality. This paperreports the effect on protein digestibility using mink(Mustela vison L) as a model animal.

MATERIALS AND METHODSProduction of fish meal and fish feed at pilotscaleTwo fish meals were produced at 70 ◦C (FM1)and 100 ◦C (FM2). Fresh (total volatile nitrogenof ∼200 mg kg−1) whole Norwegian spring spawningherring (Clupea harengus L) was heated to coagulatethe protein in a screw cooker with a heating watertemperature of about 85 ◦C for 20–25 min, duringwhich the temperature inside the fish rose from about5 to 70–80 ◦C. The fish was pressed in a twin screwpress, and the press liquor separated into oil andstick water. The stick water was concentrated in afalling film evaporator (temperature: 60 to 80 ◦C;residence time: 30 to 60 min) to a dry matter (DM)content of 250–300 mg kg−1. The press cake and theconcentrated stick water were mixed and antioxidantwas added (300 mg ethoxyquin per kilogram DM).In order to avoid deterioration of the fish materialprior to drying, the moisture content was reducedto 400–420 mg kg−1 in an indirect steam Rotadiscdryer run at low temperature (exit meal temperature<60 ◦C). The semi-dried material was chilled on

ice and stored at −30 ◦C pending drying. Low-temperature drying (FM1) was performed in a pilot-scale Ultra-Rotor grinding air drier: drying time20 to 30 s; outlet meal temperature about 70 ◦C.High-temperature drying (FM2) was performed ina pilot-scale indirect steam Rotadisc dryer. In order toincrease the heat load, the meal outlet was closed andthe meal held in the drier in excess of 1 h. The outletmeal temperature was about 100 ◦C. The dried mealsfrom both dryers were filled in polyethylene bags andstored at ambient temperature pending further usage.

Feeds for salmon and trout/seabass were processedusing a Wenger (TX-52) co-rotating twin screw pilot-scale extruder according to a factorial 2 × 3 design,with the fish meal types (FM1 and FM2) and theextruder condition (ET1, ET2 and ET3) as factors.The extrusion settings were varied to expose the feed todifferent temperatures (Table 1) and were as follows:

• increased thermal energy from ET1 to ET2 to ET3by the use of more steam and less water in thepre-conditioner;

• increased mechanical energy input from ET1 toET2 to ET3 by adding less water to the extruder;

• replaced double flight, three-quarter pitch screws inthe cooking zone (zone 6 and 7) (ET1) with doubleflight, three-quarter pitch, cut flight screws (ET2and ET3) in order to increase mechanical energyinput and increase cross-channel mixing.

The speed of the paddles in the pre-conditioner, theextruder screw speed and the feed flow rate duringall the extrusion experiments were 200 rpm, 400 rpmand 150 kg h−1 (DM) respectively. Temperature wasmeasured at the outlet of the pre-conditioner, on thesurface of the feed mass inside the extruder and in

Table 1. Extrusion and dryer conditions in pilot installation

Extruder Dryer

T ( ◦C)

PCa T ( ◦C) Z3b Z4 Z6 Z7 P (bar) Z7 Loadc (%) Moistured (%) T ( ◦C) Time (min)

FM1ET1 PSA 63 100 93 60 70 18 19 21.0 70 34FM1ET2 PSAe 74 110 107 65 75 14 28 17.5 60 27FM1ET3 PSA 94 123 126 81 82 18 36 12.0 60 14FM2ET1 PSA 63 101 94 59 70 18 18 22.0 70 33FM2ET2 PSA 74 110 107 66 75 15 28 17.5 60 30FM2ET3 PSA 94 126 129 88 81 18 33 12.3 60 14FM1ET1 PTR/PSBf 63 103 98 69 71 18 22 24 80 45FM1ET2 PTR/PSB 73 111 109 65 79 16 32 18.7 70 28FM1ET3 PTR/PSB 93 122 129 89 88 25 42 12.0 60 15FM2ET1 PTR/PSB 64 100 93 66 72 22 20 24.3 80 48FM2ET2 PTR/PSB 73 110 107 66 79 16 31 20.3 70 28FM2ET3 PTR/PSB 93 124 128 96 89 25 42 11.2 60 15

a PC: pre-conditioner.b Z: zone.c Motor load.d The moisture content is measured in the extrudate. The moisture content in the feed mass inside the extruder is approximate 2–4% higher.e PSA: salmon feed.f PTR/PSB: trout and sea bass feed.

776 J Sci Food Agric 83:775–782 (online: 2003)

Processing effects on protein quality of extruded fish feed

the hot air entering the dryer. The residence timein the pre-conditioner was between 50 and 60 s. Theextruder consisted of seven zones numbered from inletto outlet. The residence time in zones 1 to 5 was lessthan 5 s and in zones 6 and 7 between 5 and 10 s. Theextruded pellets were dried in a Paul Klockner (Type200.2) carousel dryer to a moisture content of about7% and thereafter coated in a Wenger fat applicatoruntil the desired fat content was achieved. The feedfor trout/sea bass (PTR/PSB) contained less fat thanthat for salmon (PSA) but had otherwise similarcompositions (Table 2). It was necessary, therefore,to add less oil to the feed for trout/seabass prior toextrusion than to the feed for salmon in order toprevent the feed for the former from floating in thewater when fed.

Furthermore, the feeds for salmon were processedthrough a die diameter ϕ of 3.5 mm and those fortrout/sea bass were 2.5 mm. Thus, for each type offeed, ie each type of fish meal and extruder condition,two feeds were produced, one for salmon with a higherfat content and a pellet size of 3.5 mm, and one fortrout/sea bass with a lower fat content and a pellet sizeof 2.5 mm; all together there were 12 different feeds.

Production of fish meal and fish feed atcommercial scaleTwo different grades of fish meal were obtainedon the regular market. A Norse LT 94 batch(hereafter referred to as CFM1) was produced ata factory equipped and certified for production of

low-temperature fish meal (Norsildmel AL, Bergen,Norway). A NorSeaMink batch (hereafter referredto as CFM2) of a lower quality standard wasproduced at another factory under processing con-ditions that allowed the meal to be exposed tohigher temperatures. Both meals contained 200 mgethoxyquin per kilogram as antioxidant. The mealswere stored in big bags in normal warehouses pendingusage.

CFM1 and CFM2 were used to produce fish feedsin a commercial plant (ASA srl–Agridea, ColognaVeneta, VR, Italy), according to the same factorialdesign as applied in the pilot study. Feeds for salmonand trout/sea bass were extruded simultaneously underthe same conditions and the extruded pellets were thencoated with different amounts of fish oil according tothe target species. The feeds were processed in a single-screw X-165 extruder (Wenger, Sabetha, KS, USA),dried in a model 1002 (series 6/B) belt conveyordryer (Wenger, Sabetha, KS, USA) to a moisturecontent of about 9% and finally coated with oil in avacuum oil coater (Lamec srl, Cittadella, PD, Italy)until the desired fat content was achieved. Threeextrusion conditions were designed mimicking thoseused in the pilot study, as detailed in Table 3. Theformulae for the feeds for salmon and trout/sea bassapproximated the composition of the feeds used in thepilot study, to achieve 51% and 55% protein, 25%and 18% fat and about 9% and 10% carbohydraterespectively.

Table 2. Composition of feeds for salmon and for trout and sea bass

Species of fish Salmon Trout/sea bass

Fishmeal/extruder conditions FM1ET1 FM2ET1 FM1ET1 FM2ET1FM1ET2 FM2ET2 FM1ET2 FM2ET2FM1ET3 FM2ET3 FM1ET3 FM2ET3

Pellett size (mm) 3.5 3.5 2.5 2.5Composition (g kg−1)

Fish meal FM1 651 716Fish meal FM2 651 716Capelin oil 208 205 130 125Wheat meal 127 130 140 145Vitamin Ca 01 01 01 01Vitamin/mineralMixtureb 3 3 3 3Celitec 10 10 10 10

Content (g kg−1)d

Moisture 54 47 53 54Protein 507 (536) 512 (537) 556 (587) 554 (586)Fat 255 (270) 257 (270) 181 (191) 186 (197)ASH 102 (108) 98 (103) 112 (118) 109 (115)Carbohydratee 82 (66) 86 (90) 98 (103) 97 (103)

a Provide per kg of feed, vitamin C, 250 mg.b Provide per kg of feed, vitamin A, 3000 IU; vitamin D, 1600 IU; vitamin E, 160 IU; vitamin K, 12 mg; thiamin, 12 mg; riboflavin, 24 mg; pyridoxin,12 mg; niacin, 120 mg; folate, 6 mg; vitamin B12, 0.024 mg; biotin, 0.6 mg; Ca-pantothenate, 48 mg; Cu, 3 mg; I, 2.4 mg; Fe, 24 mg; Mn, 21 mg; Zn,30 mg; Se, 0.1 mg.c Source of AIA Celite 545 Puriss. Celite 545 SP 100% (calcinated diatomit) SiO2 <60%, Quartz < 3%. Laboglass as, PO Box 364, N-4033 Forus,Norway.d Values in brackets calculated on DM basis.e Calculated by difference.

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Table 3. Extrusion and dryer conditions in commercial plant

Extruder Dryer

T ( ◦C)

PCa T ( ◦C) Z3b Z4 Z6 Z7 P (bar) Z7 Loadc (%) Moistured (%) T ( ◦C) Time (min)

CFM1ET1 66 68 80 64 89 11 65 24.5 74 22CFM1ET2 77 79 85 84 90 15 77 21.5 74 18CFM1ET3 93 103 110 111 89 19 83 19.5 74 14CFM2ET1 66 68 80 64 89 11 65 24.5 74 22CFM2ET2 77 79 85 84 90 15 77 21.5 74 18CFM2ET3 93 103 110 111 89 19 83 19.5 74 14

a PC: pre-conditioner.b Z: zone.c Motor load.d The moisture content is measured in the extrudate. The moisture content in the feed mass inside the extruder is approximate 2–4% higher.

Chemical and technical analysesIn fish meal, feed and mink faeces protein (N × 6.25)was analysed by the Dumas method (ISO/CD 15670).Moisture (ISO 6496-1983) and ash (ISO 5984-1978)were measured gravimetrically after drying for 4 hand after combustion for 16 h at 550 ◦C respectively.Fat content was determined according to the Soxhletmethod, in fish meal as described in AOCS Ba-38and in feed according to 84/4/EØF. The content ofwater-soluble protein was determined by extractingfish meal in boiling water, filtering and determiningnitrogen content in the supernatant. Total volatilenitrogen (TVN) in fish meal was determined bydistillation.10 Fluorodinitrobenzene (FDNB)-reactivelysine was determined by Salamon & Seaber, UK,according to Carpenter.11 A friability test was usedto measure the strength of the pellets in pneumatictransport. Into a Holmen Pellet Tester, 100 g pelletswere placed in the tester’s hopper and circulatedfor 60 s for 3.5 mm pellets and for 30 s for 2.5 mmpellets. The friability (expressed as a percentage) wascalculated by dividing the weight of sample remainingin the 3 mm sieve tray for 3.5 mm pellets or in the2 mm sieve tray for 2.5 mm pellets by the weight of theoriginal sample placed in the hopper.

Determination of true protein digestibility in minkTrue protein digestibility in fish meal and fish feedwas determined in mink. The fish meals were mixedwith heat-treated maize starch, soybean oil, vitaminsand minerals and water was added to the mixtureto form a dough. This was fed to four (pilot study)or six (commercial feed) mink at a rate of 50 g DMper mink once per day for 7 days. During the last4 days, faeces were collected quantitatively and frozenat −20 ◦C. The frozen faeces were lyophilised at aplate temperature of 30 ◦C. The fish feed was ground,mixed with water to a dough and fed as describedfor fish meal. Samples of homogenized freeze-driedfaeces and composite samples of feed were analysed fornitrogen and protein calculated as N × 6.25. Proteindigestibility was calculated as grams digested per

1000 g consumed according to the following equation:

Protein digestibility = Ni − (Nf − Ne)

Ni× 1000

where Ni (g) is the amount of nitrogen consumed,Nf (g) is the amount of nitrogen in the faeces andNe = 2.780 mg endogenous faecal nitrogen kg−1 feedDM.12

STATISTICAL METHODSComparison of digestibility in fish meals was doneaccording to Students t-test13 with four and sixreplicates per treatment for pilot and commercialmeals respectively. Statistical differences between feedsamples were tested according to a two (fish meal)times three (extruder conditions) factorial design13

with four and six replicates per treatment for pilot andcommercial feeds respectively.

RESULTSTechnical quality of the feedsAll extrusion conditions gave feed of acceptabletechnical quality as measured by friability, whichvaried between 91.7 and 99.9%.

Pilot-scale productionFish mealChemical composition and protein digestibility ofthe fish meals produced at the pilot scale areshown in Table 4. Prolonged time of drying athigher temperature reduced moisture content inFM2 compared with FM1. Higher temperatureexposure also led to reduced protein content ona DM basis, which evidently was due to loss ofvolatile nitrogenous bases. Furthermore, FM2 hada higher fat content and a lower content of water-soluble protein than FM1, though the heat treatmentevidently had no effect on the content of ash. Thehigher temperature exposure significantly (P < 0.005)



Table 4. Chemical composition (g kg−1) and true protein digestibility

(TDp) (grams digested per kilogram consumed, average ± sd) of fish

meal produced at pilot scale at low (FM1) and high (FM2) temperature

exposure

FM1 FM2

Composition As is In DM As is In DM

Moisture 63 17Protein (N × 6.25) 745 795 769 782Fat 71 76 88 90Ash 135 144 139 141Water-soluble protein

(g 16 g−1 N)23.5 18.3

TDp 929 ± 10 905 ± 2

reduced true protein digestibility by 24 ± 5 (average ±sd) units.

Fish feedNo significant (P = 0.193) difference in proteindigestibility between salmon and trout/seabass feedwas found. Therefore, the statistics were calculatedfor both types of feed combined (ie with eightreplicates per treatment). Furthermore, there wasno significant (P = 0.229) interaction between fishmeal and extruder conditions, and the results of thetwo treatments are therefore presented separately inTable 5. Fish feeds based on the low-temperaturefish meal (FM1) had a 21 units and significantly(P < 0.001) higher protein digestibility than thosebased on the high-temperature fish meal (FM2). Therewere no significant (P > 0.05) differences in proteindigestibility between the extrusion conditions. Basedon the protein digestibility determined for the two fishmeals and the tabulated figure for wheat (790 g per1000 g consumed),14 fish feeds made from FM1 andFM2 were calculated to have protein digestibilitiesof 924 and 901 respectively, which are 10 and9 units higher than the figures actually measured.Thus, although no differences were found betweenextrusion conditions, there was a reduction in proteindigestibility in the course of the total process, ieextrusion, drying and oil coating.

Fish feed produced under commercial conditionsThe protein digestibility of commercial fish mealCFM1 (low temperature exposure) was 940 ± 9

(average ± sd) and that of CFM2 (high temperatureexposure) was 888 ± 6, the difference being highlysignificant (P < 0.001). It was assumed that feedsfor salmon and trout/sea bass had similar proteindigestibilities, which was confirmed by testing proteindigestibility in a few parallel samples. Proteindigestibility, therefore, was only determined in thesalmon feeds. There was no significant (P = 0.8422)interaction between fish meal and extrusion conditions(Table 5). CFM1-based feed had 41 units andsignificantly (P < 0.001) higher protein digestibilitythan CFM2-based feed. There was no significant(P > 0.05) difference in digestibility between theextrusion conditions. Protein digestibility calculatedfrom ingredient digestibility as described in the pilotproduction of feed was 936 and 885 for the CFM1 andCFM2 based feeds respectively. Thus, as found in thepilot-produced feeds, the process of feed productioncaused a reduction in protein digestibility. Contrary towhat was found in the pilot study, where the reductionwas the same for both qualities, the reduction was 11units greater for the feed based on the high-qualityfish meal than that for the feed based on the lowerquality fish meal. The content of FDNB-reactivelysine in the fish meals CFM1 and CFM2 and thewheat grain used as ingredients were 7.61 g, 7.29 gand 3.00 g respectively per 16 g N. Using these figuresand the protein contribution from each ingredient,the content of FDNB-reactive lysine was calculated to7.47 g and 7.16 g per 16 g N in CFM1 and CFM2-based feeds respectively. Thus, for both types of feed,the actual measured content was much lower than thatcalculated. On the other hand, there was no consistenteffect of fish meal quality or extrusion conditions onthe FDNB-reactive lysine content determined.

DISCUSSIONMink were used as model animals to determineprotein digestibility, because these animals showless variability than fish, and because digestibilitydetermined in mink is highly correlated with thatfound in salmon and rainbow trout.15–17 The proteindigestibility, as determined in mink, was broadly inline with other findings in the collaborative study,such as the chemical criteria of protein quality, ieracemization of aspartic acid (Luzzana U pers comm)and the formation of disulphide bonds (Jensen H

Table 5. True protein digestibility (TDp) (grams digested per kilogram consumed) and FDNB-reactive lysine (g 16 g−1 N) of salmon and trout/sea

bass feed produced from FM1 and FM2 at different extrusion conditions in the pilot plant

Type of fish meal Extrusion condition

FM1CFM1

FM2CFM2 ET1 ET2 ET3 SEMa

Pilot study TDp 913 892 904 900 904 3.2Commercial study TDp 912 871 894 900 891 3.3FDNB-reactive lysine 4.66 4.88 4.86 4.70 4.77

a SEM: standard error of treatment means = √s2/n.


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and Miller ELM pers comm), and in vitro proteindigestibility.18 The difference in protein digestibilitybetween fish meals with different thermal historieswas significant for both the pilot plant and thecommercial meals, but of different magnitude. Thus,although the difference between the two commercialfish meals (CFM1 and CFM2) was substantial andin line with previous studies,19 the difference betweenthe pilot-scale-produced meals (FM1 and FM2) wasrelatively small and less than would be expectedfrom previous studies. The pilot-plant meals weresimilar in all respects except temperature exposure.For the commercial fish meals, it cannot be excludedthat factors other than temperature may have varied,since they were produced at different factories andat different times. However, it was confirmed thatboth fish meals were produced from fish with thesame high degree of freshness. The process conditionsfor FM1 and FM2 prior to drying were identical.Although it was intended to keep the temperature low,it cannot be excluded that temperatures in part ofthe process may have been higher than intended. Inthat case, this may have had a negative effect on theprotein digestibility of FM1 but not on FM2, whichwas dried at the higher temperature. The equipmentdid not allow continuous measurement of moistureduring the drying process of FM2. It is possible thatthe temperature remained low until the meal was dry.Other parts of this project have shown that the rates ofthe reactions causing racemization of aspartic acid20

and formation of disulphide bonds (Miller ELM andJensen H pers comm), both of which are believed tohave negative effects on protein digestibility,21,22 arereduced at low moisture content. That may explainwhy FM2, despite high temperature and long time heatexposure, still had only moderately reduced proteindigestibility.

Extrusion is a complex process involving interrelatedprocess parameters, such as barrel and mass temper-ature, moisture content, feed rate and screw speed,compression, die diameter, residence time and pres-sure, in addition to type of extruder (eg single or twinscrew) and types of material,3 but temperature, shear,moisture and speed have been shown to have the great-est impact on protein quality.3,9,23 The present studyused extrusion conditions that were less harsh thanthose frequently used in previous experiments, sincethey were designed to encompass the variations used incommercial fish feed production, and varying condi-tions did not affect protein digestibility or the contentof FDNB-reactive lysine. Neither did they affect thein vitro protein digestibility determined in another partof the project;18 nor did they affect the content ofD-aspartic acid (Luzzana U pers comm) and disul-phide bonds (Miller ELM and Jensen H pers comm),which are correlated with protein digestibility.21,22

However, significant reductions in protein digestibilityand FDNB-reactive lysine content were found due tothe total process, ie extrusion, drying and oil coating.Similarly, the total process increased the content of

D-aspartic acid (Umberto U pers comm) and disul-phide bonds (Miller ELM and Jensen H pers comm),extending previous findings on chemical reactions inextrusions,6,8 and the in vitro protein digestibility18

was reduced. The complexity of the extrusion processmakes it difficult to compare the results of differ-ent studies.3 Furthermore, many previous studies maynot be of direct relevance to the present study, sincethey used vegetable proteins in food systems and didnot comprise the additional processing steps of dry-ing and oil coating. Previous studies on extrusion offeeds containing animal proteins are scarce and havegiven conflicting results. Thus, Mundheim24 foundthat extrusion of fish feed containing high levels of fishmeal and wheat did not show differences in proteindigestibility before and after extrusion, the conditionsof which are not shown. Similarly, using less rigor-ous extrusion conditions than those used in this study(62 to 75 ◦C in material temperature), Pongmaneeratand Watanabe25 did not find that extrusion processingof fish feed containing varying contents of fish mealand soybean meal affected growth and protein utiliza-tion in rainbow trout. In contrast, Bhattacharya et al26

found that increasing extrusion temperature from 100to 140 ◦C decreased the in vitro protein digestibilityof a mixture of minced fish and wheat, and Camposand Areas27 found a reduction in protein quality ofdefatted lung flour when decreasing the moisture con-tent from 30 to 18% at 130 ◦C. It thus appears thatstudies showing negative effects of varying extrusionconditions used conditions outside the range appliedin this study. In line with the present study, Ljøkjeland Skrede28 found a significant reduction in proteindigestibility due to extrusion of a mixture of high-quality fish meal and wheat grain at 100 ◦C but foundno further reduction for temperature increments up to150 ◦C.

Several studies29–31 have shown that extrusion ofmaterials containing reducing sugars causes reductionsin FDNB-reactive lysine. However, the feeds usedin the present study, consisting of wheat grain andfish meal, did not contain reducing sugars in theiroriginal form. Since it is unlikely that other reducingcompounds in the feed could account for the loss inFDNB-reactive lysine, reducing carbohydrates musthave been formed in the extruder.32–36 The formationof reducing carbohydrates from starch depends on theextruder conditions, and this increases with increasingtemperature and moisture content.32,35 The findingthat varying extrusion conditions did not affect theloss of FDNB-reactive lysine in the present studymust be due to the fact that the content of reducingcarbohydrates was not rate-limiting for the Maillardreaction that occurred in the dryer.

It appears that the reduction in protein digestibilitydue to processing fish feed in the present study wascaused by a combined effect of various chemicalreactions, ie racemization of amino acids, formation ofdisulphide bonds and Maillard reactions. In addition,there is the possibility that soluble fibres in the



wheat formed in extrusion37 may have reduced proteindigestibility.38

A significant finding in this study is that the proteinquality of the finished fish feed depends to a largedegree on the quality of the fish meal. However, thereis a reduction in protein quality of the fish meal dueto processing fish feed, and that reduction appears tobe greater for fish meal of high quality than of lowquality.

CONCLUSIONHigher temperature exposure in the processing of fishmeal reduced protein digestibility. The protein qualityof fish feed depended on the quality of the fish meal.Production of fish feed from fish meal and wheat grainby extrusion cooking combined with drying and oilcoating reduced the content of FDNB-reactive lysineand protein digestibility, and the reduction was greaterfor fish meal of higher than lower protein quality.Variation in extrusion conditions within levels usedin commercial production did not affect the proteinquality of the feed.

ACKNOWLEDGEMENTSThe study has been carried out with financial supportfrom the Commission of the European Communities,Agriculture and Fisheries (FAIR) specific RTDprogram CT96-1329 ‘Effect of processing technologyon the quality of aquaculture feeds’. The paperdoes not necessarily reflect its views and in no wayanticipates the Commission’s future policy in this area.

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Effect on protein digestibility of different processing conditions in the production of fish meal and fish feed